This invention relates to thin film silicon solar cells and methods for forming particle-derived ohmic contacts with p-type silicon. A process is provided for etching the oxide or hydroxide corrosion layer from the surface of metallic particles, such as aluminum, which is shown to improve the electrical properties of the contacts to the p-type silicon.
While various methods for the production of particle derived thin-films have been reported, the thin-film application of these materials in microelectronics has been hampered by a lack of good interparticle connectivity when applied as films to substrates. This connectivity, both structural and electrical, is in many cases limited by the formation of a corrosion layer (i.e., oxide or hydroxide) contaminating the surface of the particles. These passivated surfaces generally give rise to an insulating effect electrically and an impedance in particle sintering structurally.
In order to form a thin-film for use in photoconversion and microelectronic device applications, the regions coated with particles are typically sintered to ensure electrical continuity across the feature. A reduced melting temperature for nanophase metals (C. R. M. Wronski, Brit. J. Appl. Phys., (1967)18:1731; J. F. Pocza, A. Barna and P. B. Barna, J. Phys. F. (1972)2:441) and semiconductors (A. N. Goldstein, C. M. Echer and A. P. Alivisatos, Science, (1992)256:1425; and A. N. Goldstein, Ph. D. dissertation, University of California at Berkley (1993)) has previously been shown. These disclosures are incorporated by reference herein. High temperature sintering often eliminates some of the problems associated with surface contamination. However, in the formation of nanoparticle contacts with photovoltaic semiconductors, standard high-temperature sintering often cannot be performed due to thermal limitations which are associated with the underlying layers of the device.
In view of the foregoing considerations, there is an apparent need for a low temperature method for etching the corrosion oxide or hydroxide layers. Etching improves the electrical conductivity across the feature and is useful in the formation of particle-derived ohmic contacts, such as aluminum, with a p-type silicon layer of silicon solar cells. The formation of a post-etch passivation layer impedes further corrosion and renders the particles stable to oxidizing and hydrous environments. Accordingly, a primary object of the present invention is to provide a method for etching a metallic particle surface to remove the corrosion layer to form a post-etch passivating layer on the particles, and, ultimately, to improve electrical conductivity of metal contacts.
Another object of the present invention is to provide an improved method for forming ohmic contacts derived from metallic particles, such as aluminum, with a p-type silicon layer of a silicon solar cell.
These and other objects of the present invention will become apparent throughout the description of the invention which now follows.
The present invention provides a process for etching a corrosion layer, such as oxide or hydroxide, from and concomitantly forming a passivating layer on the surface of metallic nanoparticles. A reaction mixture is prepared by dispersing, sodium hexafluoroacetylacetonate (Na(hfa)) and a metallic particle powder having oxide or hydroxide corrosion layers in hexane solvent. The mixture is allowed to react for a time sufficient to etch the oxide or hydroxide groups from the particulate surface and passivate the surfaces with (hfa). Hexane maybe evaporated from the mixture and any excess Na(hfa) separated from the reaction mixture by sublimation or rinsing with a polar aprotic solvent. In an embodiment of the present invention, aluminum particles are first etched and passivated and then used to form ohmic contacts with p-type silicon. This etching/passivation improves the electrical properties of the contact.